Optical disk apparatus for producing information recorded on a recording track

ABSTRACT

An optical disk apparatus includes a photodetector divided into eight segments which receive the laser beam reflected from the optical disk and produce signals based on the laser beam sport thereon. In this apparatus, these eight segments are combined in several patterns for the recorded information reproduction, the tracking error detection, and the focus error detection, respectively. For the recorded information reproduction, eight segments are used as combined in three segment cells RC1, RC2, and RC3 which extend in a direction parallel to each other such that the segment cell RC1 is in center and is sandwitched by other segment cell RC2 and RC3. The segment cells RC1, RC2, and RC3 are located perpendicular to the recording track of the optical disk and produce three signals C, R, and L indicative of the laser beam intensity of the laser beam spot thereon, respectively. Thus obtained signals C is multiplied by a constant K and then added to the other signals R and L. By selecting suitable value as the constant K, the crosstalk component in the reproduced signal can be reduced.

This application is a continuation of application Ser. No. 07/993,384,filed Dec. 18, 1992, now U.S. Pat. No. 5,347,504.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disk apparatus for opticallyrecording on the optical recording medium or reproducing the recordedinformation therefrom and, more particularly, to an optical diskapparatus for optically recording and reproducing an information from anoptical disk.

2. Description of the Prior Art

One effective means of improving the recording density of an opticaldisk apparatus is to reduce the track pitch. Unfortunately, cross talkof the signals reproduced from adjacent tracks to a target trackincreases when the track pitch is reduced. A method for solving suchproblem, for example, which the Japanese Laid-Open Patent PublicationNo. 57-58248, published Apr. 7, 1982 proposes is described withreference to FIG. 25. When a target signal SC is reproduced from atarget track 2C by a laser spot 1C impinged thereon, the edge of thelaser spot 1C intrudes into the range of the adjacent tracks 2L and 2R.As a result, three signals are simultaneously reproduced from threetracks 2L, 2C, and 2R laser spot 1C representing different informationrecorded on tracks 2L, 2C, and 2R, respectively. Thus, the target signalSC from the target track 2C is interfered by the noise signals from theadjacent tracks 2L and 2R, suffering from a cross talk caused by thosesignals.

To eliminate this problem, additional laser spots 1L and 1R are providedto scan the adjacent tracks 2L and 2R. The signals from the adjacenttracks 2L and 2R are therefore simultaneously played back, multiplied bya given constant, and subtracted from the intended track 2C signal toreduce crosstalk interference from the adjacent tracks 2L and 2R. Theresulting signal S can be defined by the equation of

    S=SC-K×(SL+SR)

where SC is the reproduced signal from track 2C output by the firstlaser spot 1C, SL and SR are the playback signals from tracks 2L and 2Routput by laser spots 1L and 1R, respectively, and K is a constant.

However, as will be obvious, the method of the prior art as thusdescribed requires three laser spots to simultaneously read the signalsfrom the target track and the two tracks adjacent thereto. This resultsin a more complex optical system, and makes it difficult to reduce unitsize and cost, and to improve reliability.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide an imagingdevice which solves these problems.

The present invention has been developed with a view to substantiallysolving the above described disadvantages and has for its essentialobject to provide an improved optical disk apparatus.

In order to achieve the aforementioned objective, a optical diskapparatus for reproducing an information recorded on a recording trackof an optical recording medium comprises a light source means foremitting a laser beam in a first direction; a beam splitting meanslocated adjacent said laser source means for splitting said emittedlight into first, second, and third leaser beams having diffractionorders of +1, 0, and -1, respectively; a converging means locatedbetween said optical recording medium and said beam splitting means forconversing said second laser beam on said recording track and said firstand second laser beams around opposite sides of said recording track; abeam guide means located in piths of reflected laser beams from saidoptical recording medium in a second direction; a first photodetectionmeans inserted in a path of said guided second laser beam reflected fromsaid recording track passing through said converging means and said beamguide mean, said first photodetection means receiving said second laserbeam for producing focus signal and producing information signalsrepresenting said information recorded on said recording track based onsaid received reflected second laser beam; and second photodetectionmeans inserted in paths of said first and third laser beams reflectedfrom portions around opposite sides of said recording track,respectively, for producing tracking signals.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1A is a block diagram of an optical disk apparatus according to afirst preferred embodiment of the present invention,

FIG. 1B is a plan view showing three laser beam spots focused on arecording surface of the optical disk,

FIG. 2 is a plan view showing an example of photodetector used for thedetection of focus error,

FIG. 3 is a plan view showing an example of photodetector used forreproduction of the information recorded on the tracking of the opticaldisk,

FIG. 4 is an illustration of assistance in explaining the lightintensity pattern of reflected light from the optical disk when there isa pit in the adjacent track,

FIG. 5 is an illustration of assistance in explaining the lightintensity distribution on the spherical aperture plane when there is apit string on the adjacent track,

FIG. 6 is an illustration of assistance in explaining the lightintensity distribution of the reflected light from the optical disk whenthere is a plurality of pits formed on the adjacent track,

FIG. 7 is a graph showing a mathematically simulated intensitydistribution of the light reflected from the disk when there is a pitstring formed on the adjacent tracks,

FIG. 8 is a graph showing a time-base phase inversion in the crosstalkcomponent,

FIGS. 9A and 9B are graphs showing crosstalk values simulated withrespect to the constant K and experimented with a predeterminedconditions,

FIG. 10 is a block diagram showing the first photodetector, the focussignal processing unit, and the reproduced signal processing unit of theoptical disk apparatus shown in FIG. 1,

FIGS. 11 and 12 are block diagrams showing alternations of the opticaldisk apparatus shown in FIG. 10,

FIG. 13 is a block diagram of an optical disk apparatus according to asecond preferred embodiment of the present invention,

FIG. 14 is a plan view showing an example of photodetector used fortracking error detection by the push-pull method,

FIG. 15 is a block diagram showing the first photodetector, the trackingsignal processing units, and the reproduced signal processing unit ofthe optical disk apparatus shown in FIG. 13,

FIGS. 16 and 17 are block diagrams showing alternations of the opticaldisk apparatus shown in FIG. 15,

FIG. 18 is a block diagram of an optical disk apparatus according to athird preferred embodiment of the present invention,

FIGS. 19A and 19B are plan views showing examples of photodetectors usedfor the spot size detection method,

FIGS. 20A and 20B are plan views showing photodetectors used for theinformation signal reproduction according to the third embodiment,

FIG. 21 is a block diagram showing a reproduction signal processing unitshown in FIG. 18,

FIG. 22 is a block diagram showing a focus signal processing unit shownin FIG. 18,

FIGS. 23 and 24 are block diagrams showing alternations of focus signalprocessing unit shown in FIG. 22, and

FIG. 25 is an illustration of assistance in explaining the crosstalkreducing method of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1A, an optical disk apparatus according to a firstembodiment of the present invention is shown. The optical disk apparatusOA1 includes a support unit 6 for rotatably supporting an optical disk 5having a main plane formed with a track for recording the informationthereon. The optical disk apparatus OA1 further includes asemi-conductor laser source 1 for emitting a laser beam L along anoptical axis A1 and a deflection plate 2 for diffracting the emittedlaser beam Lb to produce three laser beams Lb⁺, Lb0, and Lb⁻ havingdiffraction orders of "+1", "0", and "-1", respectively.

A half mirror 3 is provided on one side of the deflection plate 2 awayfrom the laser source 1 with its main plane at a predetermined angle sothat the half mirror 3 reflects diffracted laser beams Lb⁺, Lb0, and Lb⁻toward the disk 5 along an optical axis A2, as shown in FIG. 1A. Acondenser lens 4 is provided between the optical disk 5 and the halfmirror 3 for converging the reflected laser beams Lb⁺, Lb0, and Lb⁻ tomake spots SP⁺, SP0, and SP⁻ respectively, focused on a recording planeof the optical disk 5, as shown in FIG. 1B. The spots SP⁺, SP0, and SP⁻located on positions of the target track in a pattern according to theconventional three beam detection method is shown.

A first photodetector 11 made by a photoelectric element is provided onthe optical axis A2 for receiving the laser beams Lb0 reflected from theoptical disk 5 through the condenser lens 4 and the half mirror 3. Thefirst photodetector 11 produces electric signals representing theinformation recorded in the target track based on the received laserbeam Lb0. Second and third photodetectors 7a and 7b made by aphotoelectric element are also provided on the opposite sides of thefirst photodetector 11 for receiving the laser beams Lb⁺ and Lb⁻ andproducing signals, respectively.

A tracking signal processing unit 9 is electrically connected withsecond and third photodetectors 7a and 7b for detecting the trackingerror based on signals from the photodetectors 7a and 7b. A focus signalprocessing unit 10 is provided in an electrical connection with thefirst photodetector 11 for detecting the focus error based on signalsfrom the first photodetector 11. The optical disk apparatus OA1 furtherincludes a reproduced signal processing unit 12 connected with the firstphotodetector 11. The reproduced signal processing unit 12 reproducesthe information signal based on the signals from the first photodetector 11.

In operation, the semiconductor laser source 1 emits the laser beam Lbtoward the half mirror 3 through the deflection plate 2 by which theemitted light Lb is split into three beams Lb⁺, Lb0, and Lb⁻. The halfmirror 3 reflects the diffracted laser beams Lb⁺, Lb0, and Lb⁻ towardthe condenser lens 4. The condenser lens 4 converges the three beamsLb⁺, Lb0, and Lb⁻ to make spots SP⁺, SP0, and SP⁻ on and around therecording track. The spot SP0 is focused on the target track, and spotsSP⁺ and SP⁻ are focused on opposite sides of the target track,respectively, according to the three beam detection method.

The beams Lb⁺, Lb0, and Lb⁻ are reflected by the optical disk surface 5toward the condenser lens 5 and half mirror 3. Portions of the reflectedbeams Lb⁺, Lb0, and Lb⁻ pass through the half mirror 3 and reach thephotodetectors 7b, 11, and 7a, respectively. It is to be noted that thelaser beams Lb⁺, Lb0, and Lb⁻ are applied with an astigmatism whenpassing through the half mirror 3 which is inclined with respect to theoptical axis A2 at a predetermined angle.

The second and third photodetectors 7a and 7b receive the laser beamsLb⁺ and Lb⁻ and produce tracking error signals Ta and Tb based on thereceived laser beams Lb⁺ and Lb⁻, respectively. The tracking signalprocessing unit 9 processes the tracking signals Ta and Tb to obtain atracking error signal TS by the three beam method. Since the three beammethod is well known to the personell skilled in the field of opticalrecording technology, the further explanation is omitted for the sake ofbrevity.

Referring to FIG. 2, an example of the first photodetector 11 used toreceive the laser beam Lb0 for the detection of focus error is shown.The first photodetector 11a has four square cells C1, C2, C3, and C4,each cell is close to two other cells by two neighboring side edges, asshown in FIG. 2. When the spot SPO is out of focus on the optical disk5, the laser beam Lb0 makes an oval shape spot on the photodetectivesurface of the first photodetector 11a, varying the spot area on eachcell. Then, square cells C1, C2, C3, and C4 produce focus signals F1,F2, F3, and F4 (FIG. 1A) having different strength. The focus signalprocessing unit 10 processes the focus signals F1, F2, F3, and F4 toobtain a focus error signal FS, as expressed by the following equation,

    FS=F1+F4-F2-F3                                             (1).

Referring to FIG. 3, another example of the first photodetector 11 usedfor receiving the laser beam Lb0 to produce an information signal Srepresenting information recorded on the track 5 is shown. The firstphotodetector 11b has center, right side, and left side photodetectivecells RC1, RC2, and RC3, extending in a direction parallel to eachother. The center photodetective cell RC1 has a predetermined width Wand is sandwitched by right and left side photodetective cells RC2 andRC3. The first photodetector 11b is aligned in a direction perpendicularto the tangent of recording track examined by the spot SP0. Then, thelaser beam Lb0 makes a spot on the first photodetector 11b so that itsportion corresponding to the target track is parallel to thelongitudinal direction (arrow Y) of the photodetective cells RC1, RC2,and RC3, as shown in FIG. 3. The photodetective cells RC1, RC2, and RC3produce signals C, L, and R corresponding to the spot area formedthereon, respectively. The reproduction signal processing unit 12functions to multiply signal C by a constant K, and then add theresulting product to signals R and L and obtains the reproduced signal Srepresenting the information recorded on the track examined by the laserspot SP0. This function is defined by the following equation,

    S=K×C+R+L                                            (2).

It is to be noted that the crosstalk component contained in thereproduced signal S can be reduced by setting the constant K at asuitable value, as will be described later.

Referring to FIG. 4, the spot SP0 of the laser beam Lb0 focused on thetarget track 16 of the optical disk 5 is shown. When is viewedmicrographically, the recording track 16 extends in the direction Y witha plurality of pits formed thereon. An adjacent track 15 is also formedin the optical disk 5, extending parallel to the recording track 16apart therefrom by a predetermined distance x, and is formed with aplurality of pits thereon. When the center of the spot SP0 is the zeropoint of the orthogonal coordinate system with X and Y axes, theposition of a pit P formed on the adjacent track 15 is expressed asP(x,0). Regarding the reduction in the crosstalk component, now thelight intensity distribution on the diffraction light reflected by thesingle pit P on the spherical aperture plane A of the condenser lens 4is considered.

The phase φ of the diffracted laser beam Lb0 at a point Q on thespherical aperture plane A is obtained as described below. The positionof the point Q is expressed as Q (U,V) and is away from the zero pointby distances U and V with respect to the axes X and Y, respectively. Thephase φ is defined by the following equation.

    φ=2πXU/Fλ                                    (3).

F is the focal length of the condenser lens 4 and λ is the wavelength ofthe laser beam Lb.

From the equation (3), Ur which is the value of U when the value of φ isρ is obtained by the following equations.

    π=2πXUr/Fλ                                    (3')

    Ur=πFλ/2πX=Fλ/2X                       (4)

For example, when the pit P is located in the position at X=1.6 μm andY=0 μm, the Ur is computed by the equation (4) using the values F=4 mmand λ=0.8 μm, then ##EQU1## Thus, the phase φ inverts at U=±1 mm in thisexample.

Referring to FIG. 5, the light intensity distribution resulted from theabove equations is shown. The shaded areas extending vertically on bothsides of the axis Y show the darker area compared with another area.This is because that light phase φ changes from zero to π as moving awayfrom the zero point (0,0) by distance ±Ur with respect to X axis. Thelight intensity is maximum around the Y axis, decreases gradually, andis minimum around the line where X=±Ur. Therefore, the light intensitychanges from the brighter to the darker area around the position whereX=±Ur/2. When it is darker around the Y axis, the light intensitychanges from the darker to brighter. Thus, when the pits are shifted indirection X from the optical axis A1 (0,0), an interference pattern oflight and dark bands is formed on the spherical aperture surface A indirection X, as shown in FIG. 5.

Referring to FIG. 6, the light intensity distribution when plural pitsare formed on the track 16 is shown schematically. When there are pluralpits on the track 16 in the middle band (where X=0), a light-darkpattern, which will be variously determined by the pitch of the pitstring, the pit width, pit length, and other conditions, will appear inthis direction Y. Since the crosstalk is caused by plural pits on theadjacent tracks, it can be considered that the crosstalk is made bycombining the above two states, specifically when there is one pit onthe adjacent track 15 where X is not at 0 (shown in FIG. 5), and whenthere are plural pits in direction Y on the center track 16 where X=0.The light phase φ changes from zero pint (0,0) by distance±Vr withrespect to Y axis. The darker area indicated by the shading and brighterarea are arranged in a checkered pattern, as shown in FIG. 6. Thus,brighter portions P₂ and P₃ appear around the positions (0,0) and (-Ur,-Vr), and darker portions P₁ and P₄ appear around the positions (-Ur,0)and (0,-Vr), as typically shown in FIG. 6.

Referring to FIG. 7, the results of a mathematical simulation of theintensity distribution of the light reflected from the optical disk 5when there are pits only on the adjacent tracks is shown. Comparingportions P1 and P2, the areas corresponding to the phase inversionrelationship described above, we can see that the portion P2 is convexrelative to the concave shape of the portion P1, and that a similarrelationship is observed in portions P3 and P4. This graph shows thatthere is an actual inversion in the light intensity distribution, andthat the mechanism described above is valid. Because there is aquantifiable inversion of values (graphically expressible as sinkagesand lands) in the light intensity distribution on the lines X=0 andX=Ur. The ratio between the width of the center photodetective cell RC1and the laser beam diameter (W/d) affects the crosstalk component. Withthe above mechanism, however, a large normal crosstalk component will bepresent in the center photodetective cell RC1, and a large inversecrosstalk component will be present in the two side photodetective cellsRC2 and RC3, and the crosstalk cancellation effect of the mechanism isthus enhanced. Furthermore, the crosstalk cancellation effect of themechanism is still enhanced even when a large inverse crosstalkcomponent is present in the first photodetective cell RC1 and a largenormal crosstalk component is present in the two side cells RC2 and RC3.This crosstalk cancellation effect is reduced when both normal andinverse crosstalk components are mixed in all three photodetective cellsRC1, RC2, and RC3. This may occur when the dividing lines between thethree photodetectors in the three-part photodetection means are setaround X=Ur. The ideal position for these dividing lines can bequantified as approximately half of Ur, as will be described later.

Referring to FIG. 8, a time-base phase inversion in the crosstalkcomponent is shown. The line DO shows the time-base phases on the linewhere X=0, and the line DU on the lines where X=±Ur. Thus, the time-basephase inversion in the crosstalk component is equal to the variation inthe total light intensity, corresponding to this light intensitydistribution on positions where X=0 and X=±Ur. The crosstalk componentcan therefore be reduced by adding the signals together, the targetedsignal components have the same phase at X=0 and X=Ur and are thereforecomplementary, but the crosstalk components cancel each other out andcrosstalk is therefore reduced. In addition, the effect of thisoperation can be further enhanced by setting the photodetector dividinglines to obtain the one multiplied signal and the added signals so thatthe crosstalk component is minimized.

Referring to FIG. 9A, crosstalk values simulated with respect to theconstant K is shown. The following values were used in the simulation:wavelength of laser beam Lb=780 nm, aperture ratio NA of the condenserlens 4=0.53, track pitch=1.6 μm, and pit pitch is 1.8 μm. The crosstalkis minimized when the constant K=approximately 1.2.

Referring to FIG. 9B, the results of one experiment with an optical diskapparatus according to the present invention are shown. The followingvalues were used in the experiment: wavelength of laser beam Lb=780 nm,aperture ratio NA of the condenser lens 4=0.5, track pitch=0.9 μm, andpit pitch=approximately 1.8 μm. The crosstalk was minimized in thisexperiment with K=approximately 0.3, reducing the crosstalk by more than12 dB. It was thus confirmed that the method of the invention iseffective in both theory and in practice. The ratio between the width ofthe center photodetective cell RC1 and the laser beam diameter (W/d)affects the crosstalk component. With the above mechanism, however, whena large normal crosstalk component will be present in the centerphotodetective cell RC1, and a large inverse crosstalk component will bepresent in the two side photodetective cells RC2 and RC3, the crosstalkcancellation effect of the mechanism is thus enhanced. Furthermore, thecrosstalk cancellation effect of the mechanism is still enhanced evenwhen a large inverse crosstalk component is present in the centerphotodetective cell RC1, and a large normal crosstalk component ispresent in the two side photodetective cells RC2 and RC3. This crosstalkcancellation effect is reduced when both normal and inverse crosstalkcomponents are mixed in all three photodetective cells RC1, RC2, andRC3. This may occur when the dividing lines between the threephotodetective cells RC1, RC2, and RC3 in the first photodetector 11bare set around X=Ur. The ideal position for these dividing lines can bequantified as approximately half of Ur.

By thus shifting the position of the dividing line used in thethree-part photodetection means from the crosstalk phase inversionposition, crosstalk can be reduced under a wide range of pitarrangements, track pitches, and other optical conditions. In addition,because the appearance of crosstalk will be different when one of thephotodetective cells RC1, RC2, and RC3 is eliminated, a similareffectiveness can be obtained under pit arrangement conditions otherwiseresistant to crosstalk reduction.

Referring to FIG. 10, the first photodetector 11, the focus signalprocessing unit 10, and the reproduced signal processing unit 12according to the first embodiment of the present invention is shown. Thefirst photodetector 11 has a construction combining the examplephotodetectors 11a and 11b previously described with reference to FIGS.2 and 3. The first photodetector 11 includes eight rectangularphotodetective cells Ca, Cb, Cc, Cd, Ce, Cf, Cg, and Ch, extending in adirection. The photodetective cells Ca, Cb, Cc, and Cd are arranged toclose to each other side by side and are aligned to form a first row.The photodetective cells Ce, Cf, Cg, and Ch are also arranged to closeto each other side by side and are aligned to form a second row. Thesetwo rows are closed to each other side by side so that the first row'scells Ca, Cb, Cc, and Cd oppose to the second row's cells Ce, Cf, Cg,and Ch, respectively, as shown in FIG. 10. These eight photodetectivecells Ca, Cb, Cc, Cd, Ce, Cf, Cg, and Ch produce signals Sa, Sb, Sc, Sd,Se, Sf, Sg, and Sh based on the laser beam Lb0 focused thereon.

In thus constructed first photodetector 11, the combinations ofphotodetective cells Ca and Cb can be used for the photodetective cellC1 of the photodetector 11a which is used for producing a focus signal.Similarly the combination of cells Cc and Cd, Ce and Cf, and Cg and Chcan be used for photodetective cells C2, C3, and C4, respectively.Furthermore, the combinations of photodetective cells Cb, Cc, Cf, and Cgcan be used for the center photodetective cell RC1 of the photodetector11b which is used for producing the reproduced signal S. Similarly, thecombination of photodetective cells Ca and Ce can be used for the leftside photodetective cell RC2, and, cells Cd and Ch for the right sidecell RC3.

The focus signal processing unit 10 includes a first adder 20 havingfour input ports connected with the cell Ca for receiving the signal Sa,the cell Cb for receiving the signal Sb, the cell Cg for receiving thesignal Sg, and the cell Ch for receiving the signal Sh, respectively,and an output port for transmitting an added signal thereof. A secondadder 21 having four input ports connected with the cell Cc forreceiving the signal Sc, the cell Cd for receiving the signal Sd, thecell Ce for receiving the signal Se, and the cell Cf for receiving thesignal Sf, respectively, and an output port for transmitting an addedsignal thereof. The focus signal processing units 10 further includes asubtracter 22 having a positive input port connected to the output portof the first adder 20 and a negative input port connected to the outputport of the second adder 21.

The combinations of photodetective cells Ca and Cb, the cells Cc and Cd,the cells Ce and Cf, and the cells Cg and Ch are equivalent to thephotodetective cells C1, C2, C3, and C4 of the photodetector 11a,respectively, as described previously. Therefore, the output signal fromthe first adder 20 can be expressed as: ##EQU2## The output signal fromthe second adder 21 can be expressed as: ##EQU3## Then, subtraction ofthe signals by the subtracter 22 can be expressed as: ##EQU4## Thus, thefocus signal FS is obtained.

The reproduced signal processing unit 12 includes a third adder 23having four input ports connected with the cell Ca for receiving thesignal Sa, the cell Cd for receiving the signal Sd, the cell Ca forreceiving the signal Se, and the cell Ch for receiving the signal Sh,respectively, and an output port for transmitting an added signalthereof. A fourth adder 24 having four input ports connected with thecell Cb for receiving the signal Sb, the cell Cc for receiving thesignal Sc, the cell Cf for receiving the signal Sf, and the cell Cg forreceiving the signal Sg, respectively, and an output port fortransmitting an added signal thereof. A multiplier 25 has an input portconnected the output port of the fourth adder 24 for receiving the addedsignal therefrom and an output port for transmitting the received addedsignal multiplied by a predetermined constant value K. The reproducedsignal processing units 12 further includes a fifth adder 26 having twoinput ports connected to the output ports of the third adder 23 and thefourth adder 24, respectively.

The combinations of photodetective cells Cb, Cc, Cf, and Cg, the cellsCa and Ce, and the cells Cd and Ch are equivalent to the cells RC1, RC2,and RC3, respectively, as described previously. Therefore, the outputsignal from the third adder 23 can be expressed as: ##EQU5## The outputsignal from the fourth adder 24 can be expressed as: ##EQU6## The outputsignal from the multiplier 25 can be expressed as:

    (Sb+Sc+Sf+Sg)×K=K×C                            (11).

Then, addition of the signals by the fifth adder 25 can be expressed as:

    (11)+(10)=K×C+R+L=S                                  (12).

Thus, the reproduction signal S is obtained. It is to be noted that themultiplier 25 can be connected with the third adder 23 instead of thefourth adder 24 by selecting any suitable value as the constant K. Then,the same result expressed by the equation (12) can be obtained.

Referring to FIG. 11, an alternation of the first embodiment shown inFIG. 10 is shown. This alternation of the first embodiment has theconstruction similar to that of the first embodiment of FIG. 10.However, the first adder 20 has only two input ports connected with thecells Cb and Cg, respectively, instead of four input ports (two portsfor cells Ca and Ch are deleted from that shown in FIG. 10). The secondadder 21 also has only two input ports connected with the cells Cd andCf, respectively, instead of four input ports (two ports for Cc and Ceare deleted from that shown in FIG. 10). Since the four photodetectivecells are enough to detect the focus error, as described in the above,the first adder 20 produces an added signal of Sb and Sg as the addedsignal of F1 and F4 which can be expressed as:

    F1+F4=Sb+Sg                                                (6').

The second adder 21 produces an added signal of Sc and Sf as the addedsignal of F2+F3 which can be expressed as:

    F2+F3=Sc+Sf                                                (7').

Then, subtraction of the signals by the subtracter 22 can be expressedas:

    FS=Sb+Sg-(Sc+Sf)                                           (8').

Thus, in this alternation of the first embodiment, since the focussignal FS can be obtained by the photo detective cells Cb, Cc, Cf, andCg which correspond to the center cell RC1 of the photodetector 11a, itis possible to construct the optical disk apparatus OA1 more simplercompared with the that shown in FIG. 10.

Referring to FIG. 12, another alternation of the first embodiment shownin FIG. 10 is shown. This alternation of the first embodiment has theconstruction similar to that of the apparatus shown in FIG. 11. However,the first adder 20 has only two input ports connected with the cells Caand Ch instead of Cb and Cg, respectively. The second adder 21 also hasonly two input ports connected with the cells Cd and Ce instead of Ccand Cf, respectively. Thus, the first adder 20 produces an added signalof Sa and Sh as the added signal of F1 and F4, which can be expressedas:

    F1+F4=Sa+Sh                                                (6").

The second adder 21 produces an added signal of Sd and Se as the addedsignal of F2+F3 which can be expressed as:

    F2+F3=Sd+Se                                                (7").

Then, subtraction of the signals by the subtracter 22 can be expressedas:

    FS=Sa+Sh-(Sd+Se)                                           (8").

Thus, in this alternation of the first embodiment, since the focussignal FS can be obtained by the photo detective cells Ca, Cd, Ce, andCh which correspond to the side cells RC2 and RC3 of the photodetector11a, it is possible to construct the optical disk apparatus OA1 moresimpler compared with the that shown in FIG. 10.

Referring to FIG. 13, an optical disk apparatus according to a secondembodiment of the present invention is shown. The optical disk apparatusOA2 is used for the tracking error detection by the push-pull method.The optical disk apparatus OA2 has a construction similar to that of theapparatus OA1. The optical disk apparatus OA2 further includes acollimating lens 32 for causing the emitted laser beam Lb to becomeparallel. A first half mirror 33 is provided on one side of thecollimating lens 32 away from the laser source 1 and in the path of thecollimated laser beam Lb to reflect a portion of collimated laser beamLb toward the disk 5 along an optical axis A2, as shown in FIG. 1. Afirst condenser lens 4 is provided between the first half mirror 33 andthe optical disk 5 for converging the reflected laser beam Lb to makespot SP focused on a recording surface R of the optical disk 5.

On the opposite side of the half mirror 33, a second half mirror 34 isprovided in the path of the laser beam Lb reflected from the opticaldisk 5 through the first condenser lens 4 and the first half mirror 33,so that the second half mirror 34 reflects a portion of the reflectedlaser beam Lb along an optical axis A3 and allows the rest portion ofthe reflected laser beam Lb therefrom. Thus, the laser beam Lb reflectedfrom the optical disk 5 is divided in two routes along the optical axesA2 and A3, respectively.

Beside the second half mirror 34, the first photodetector 11 is providewith its photodetective surface across the optical axis A3 to receivethe divided laser beam Lb to produce electric signals. A tracking signalprocessing unit 40 and the reproduction signal processing unit 12 areelectrically connected with the first photodetector 11. The trackingsignal processing unit 40 processes the tracking signals and detects thetracking error by the push-pull method. Since the push-pull method iswell known to the personell skilled in the field of optical recordingtechnology, the detailed explanation is omitted for the sake of brevity.

Referring to FIG. 14, an example of the first photodetector 11 used fortracking error detection by the push-pull method is shown. The firstphotodetector 11c has two photodetective cells C1' and C2' arrangedclose to each other side by side, as shown in FIG. 14. Thephotodetective cells C1' and C2' produce tracking signals Ta, and Tbbased on the portions laser beam Lb focused thereon, respectively. Whenthe laser spot Sp is out of the recording track on the optical disk 5,the light density of laser beam Lb received on the photodetective cellsC1' and C2' is uneven with respect to a direction perpendicular to thetrack direction. Therefore, the tracking signal processing unit 40 canobtains the tracking error signal TS based on the tracking signals Taand Tb by the push-pull method, as expressed by the following equation,

    TS=Ta-Tb                                                   (13)

For production of the reproduced signal S, the same patter of thephotodetector 11b shown in FIG. 3 is employed. Thus, the firstphotodetector 11 has a construction combined by these two examplephotodetectors 37' and 11b, and will be described later with referenceto FIG. 16.

Referring back to FIG. 13, on the side of the second half mirror 35 awayfrom the first half mirror 33, a second condenser lens 35 having a focuspoint f is provided in alignment with the optical axis A2 for convergingthe laser beam Lb. On the opposite side of the condenser lens 35, asecond photodetector 38 is provided with it photodetective surfaceacross the laser beam Lb before the focus point f by a predetermineddistance for producing electric signals.

Between the second condenser lens 35 and the second photodetector 38, athird half mirror 36 is further provided in the path of the laser beamLb to divide the laser beam Lb passed through the second condenser lens35, so that the third half mirror 36 reflects a portion of the laserbeam Lb passed though the second condenser lens 5 along an optical axisA4 and allow the rest portion of the laser beam Lb to pass toward thesecond photodetector 38. Thus, the laser beam Lb reflected from theoptical disk 5 is divided in three routes along the optical axes A2, A3,and A4, respectively.

Beside the third half mirror 36, a third photodetector 39 is providedwith its photodetective plane across the optical axis A4 to receive thedivided laser beam Lb after the focus point f by the a predetermineddistance for producing electric signals based on the received laser beamLb. A focus signal processing unit 42 is electrically connected withsecond and third photodetectors 38 and 39 for producing focus errorsignal FS by the spot size detection method. Since the spot sizedetection method is well known to the personell skilled in the field ofoptical recording technology, the further explanation is omitted for thesake of brevity.

In operation, the tracking signal processing unit 40 produces thetracking error signal TS based on the tracking signals produced by thefirst photodetective by the Push-pull method. The reproduced signalprocessing unit 12 produces the reproduced signal based on the signalsproduced by the first photodetector 11 as described in the above. Thefocus signal processing unit 42 produces the focus error signal FS basedon the signal produced by second and third photodetectors 38 and 29 fordetection of the focus error by a conventional method.

Referring to FIG. 15, the first photodetector 11, the tracking signalprocessing units 40, and the reproduced signal processing unit 12according to the second embodiment of the present invention is shown.

The tracking signal processing unit 40 has a construction similar tothat of the focus signal processing unit 10 of the first embodiment.However the first adder 20 are connected with the cells Cc, Cd, Cg, andCh. The second adder 21 are connected with the cells Ca, Cb, Ce, and Cf.Then, the tracking signal processing unit 40 can obtain the trackingsignal TS, as expressed by the equation of, ##EQU7##

Referring to FIG. 16, an alternation of the second embodiment is shown.This alternation of the second embodiment has the construction similarto that shown in FIG. 15. However, the first adder 20 is connected withonly two cells Cc and Cg. The second adder 21 also connected with onlytwo cells Cb and Cf. Then, the tracking signal processing unit 40 canobtain the tracking signal TS, as expressed by the equation of,

    TS=(Sb+Sf)-(Sc+Sg)=Ta-Tb                                   (15).

Thus, the tracking error signal TS can be obtained by the photodetective cells Cb, Cc, Cf, and Cg which correspond to the center cellRC1 of the photodetector 11a, resulting in a simpler construction.

Referring to FIG. 17, another alternation of the second embodiment isshown. In this alternation, the first adder 20 is connected with onlytwo cells Cd and Ch. The second adder 21 also connected with only twocells Ca and Ce. Then, the tracking signal processing unit 40 can obtainthe tracking signal TS, as expressed by the equation of,

    TS=(Sa+Se)-(Sd+Sh)=Ta-Tb                                   (16).

Thus, the tracking error signal TS can be obtained by the photodetective cells Ca, Cd, Ce, and Ch which correspond to the side cell RC2and RC3 of the photodetector 11a, resulting in a simpler construction ofthe apparatus OA2.

Referring to FIG. 18, an optical disk apparatus according to a thirdembodiment of the present invention is shown. The optical disk apparatusOA3 is used for the focus error detection by the spot size detectionmethod. The optical disk apparatus OA3 has a construction similar tothat of the apparatus OA2 of FIG. 13. Second and third photodetectors 38and 39 of FIG. 13 are replaced by photodetectors 51 and 52. Furthermore,the reproduced signal processing unit 12 and focus signal processingunit 42 are replaced by alternative units 60 and 70, respectively, bothof which are connected with the photodetectors 51 and 52.

Referring to FIGS. 19A and 19B, examples of second and thirdphotodetectors 51 and 52 used for the spot size detection method areshown. The second photodetector 51' has three rectangular cells B1, B2,and B3, extending parallel to each other. The cells B1 is arrangedbetween the cells B2 and B3 as been sandwitched therebetween side byside. The cells B1, B2, and B3 produce signals SB1, SB2, and SB3 basedon the spot of laser beam Lb thereon, respectively. Similarly, the thirdphotodetector 52 has three photodetective cells B1', B2', and B3' forproducing signals SB1', SB2', and SB3' respectively.

When the laser spot Sp is out of focus over the optical disk 5, as shownby the dot line (spot Sp') in FIG. 18, the focus point f moves away thepresent focus point f to the point f' such that the spots on the secondphotodetector 51 and third photodetector 39 changes accordingly.Therefore, the focus signal processing unit 70 can obtains the focuserror signal FS based on the focus signals SB1, SB2, SB3, SB1', SB2',and SB3', as expressed by the following equation,

    FS=SB1'-SB2'-SB3'-SB1+SB2+SB3                              (17).

Thus, the focus error signal FS can be obtained by using the signalsproduced by the all cells of second and third photodetectors 51 and 52.However, it is possible to obtain the focus error signal FS by usingcenter cells B1 and B1' only, as expressed by the following equation,

    FS=SB1'-SB1                                                (18).

The focus error signal FS also can be obtained by using side cells B1,B3, B1', and B3' only, as expressed by the following equation,

    FS=SB1+SB3-SB1'-SB2'                                       (18').

Referring to FIGS. 20A and 20B, second and third photodetectors 51 and52 used for the information signal reproduction and the focus errordetection according to the third embodiment is shown. The secondphotodetector 51 has a construction combining the example photodetectors11b and 51' shown in FIGS. 3 and 19A, respectively. The secondphotodetector 51 includes nine photodetective cells Ca1, Cb1, Cc1, Cd1,Ce1, Cf1, Cg1, Ch1, and Ci1. The photodetective cells Ca1, Cb1, and Cc1are arranged to close to each other side by side and are aligned to forma first horizontal row. Similarly, the second horizontal row is formedby cells Cd1, Ce1, and Cf1, and the third horizontal row is formed bycells Cg1, Ch1, Ci1. These three rows are arranged close to each otherside by side such that the second row is sandwitched by first and thirdrows, as shown in FIG. 20. When these three rows are assembled, each ofthree cells Ca1, Cd1, and Cg1, cells Cb1, Ce1, and Ch1, and cells Cc1,Cf1, and Ci1 forms a vertical row, respectively. These photodetectivecells Ca1, Cb1, Cc1, Cd1, Ce1, Cf1, Cg1, Ch1, and Ci1 produce signalsSa1, Sb1, Sc1, Sd1, Se1, Sf1, Sg1, Sh1, and Si1, respectively, based onthe laser beam Lb spot thereon. Similarly, the third photodetector 51includes nine photodetective cells Ca2, Cb2, Cc2, Cd2, Ce2, Cf2, Cg2,Ch2, and Ci2 which produce signals Sa2, Sb2, Sc2, Sd2, Se2, Sf2, Sg2,Sh2, and Si2, respectively.

Referring to FIG. 21, the reproduction signal processing unit 60 isschematically shown. The reproduction signal processing units 60includes a first adder 61 connected to the cells Cb1, Ce1, Ch1, Cb2,Ce2, and Ch2 for producing a first added signal AS1, which can beexpressed as: ##EQU8##

A second adder 63 is connected to the cells Ca1, Cd1, Cg1, Cc1, Cf1,Ci1, Ca2, Cd2, Cg2, Cc2, Cf2, and Ci2, respectively, for producing asecond added signal AS2, which can be expressed as: ##EQU9##

A multiplier 62 is provided in connection with the first adder 61 formultiplying the first added signal AS1 by the constant value K. A thirdadder is provided in connection with the multiplier 62 and the adders 63for obtaining a third added signal AS3, which can be expressed as:##EQU10## Thus, the reproduced signal S is obtained.

Referring to FIG. 22, the focus signal processing unit 70 isschematically shown. The focus signal processing units 60 includes afourth adder 71 connected to the cells Cd1, Ce1, Cf1, Ca2, Cb2, Cc2,Cg2, Ch2, and Ci2, respectively, for producing a fourth added signalAS4, which can be expressed as:

    AS4=Sd1+Se1+Sf1+Sa2+Sb2+Sc2+Sg2+Sh2+Si2                    (22).

A fifth adder 72 is connected to the cells Cd2, Ce2, Cf2, Ca1, Cb1, Cc1,Cg1, Ch1, and Ci1, respectively, for producing a fifth added signal AS5,which can be expressed as:

    AS5=Sd2+Se2+Sf2+Sa1+Sb1+Sc1+Sg1+Sh1+Si1                    (23).

A subtracter 73 is provided in connection with fourth and adders 71 and72 for producing a subtracted signal SS by subtracting the fifth addedsignal AS5 from the fourth added signal AS5, expressed as:

    SS=AS4-AS5=FS                                              (24).

Thus, the focus error signal FS is obtained.

Referring to FIG. 23, an alternation of the focus signal processing unit70 of FIG. 22 is shown. In this alternative focus signal processing unit70, the fourth adder 71 is connected to the center cells Cd1, Ce1, andCf1 for producing the fourth added signal AS4, expressed as:

    AS4=Sd1+Se1+Sf1                                            (25).

The fifth adder 72 is connected to only the center cells Cd2, Ce2, andCf2 for producing the fifth added signal AS5, expressed as:

    AS5=Sd2+Se2+Sf2                                            (26).

The substracter 73 produces the subtracted signal SS by subtracting thefifth added signal AS5 from the fourth added signal AS5, as describedabove. Thus, the focus error signal FS is obtained by using only thecenter cells of second and third photodetectors 51 and 52.

Referring to FIG. 24, another alternation of the focus signal processingunit 70 is shown. In this alternative focus signal processing unit 70,the fourth adder 71 is connected to only the side cells Ca2, Cb2, Cc2,Cg2, Ch2, and Ci2 for producing the fourth added signal AS4, expressedas:

    AS4=Sa2+Sc2+Sc2+Sg2+Sh2+Si2                                (27).

The fifth adder 72 is connected to only the center cells Ca1, Cb1, Cc1,Cg1, Ch1, and Ci1 for producing the fifth added signal AS5, which can beexpressed as:

    AS5=Sa1+Sb1+Sc1+Sg1+Sh1+Si1                                (28).

The subtracter 73 produces the subtracted signal SS as described above.Then, the focus error signal FS is obtained by using only the side cellsof second and third photodetectors 51 and 52.

As a result, an optical disk apparatus according to the presentinvention can reduce crosstalk in a playback signal using a singleoptical beam, and it is therefore no longer necessary to increase thenumber of semiconductor lasers to improve playback performance.Furthermore, the optical disk apparatus according to the presentinvention, it is also possible to detect tracking error and focus errorat the same time of playing back the information recorded on the opticaldisk without any apparatus for detection of tracking error and focuserror separately.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. An optical disk apparatus for reproducing aninformation recorded on a recording track of an optical recording mediumcomprising:a light source means for emitting a laser beam having a firstpath in a first direction; a beam collimation means located in saidfirst path for collimating said emitted laser beam; a first convergingmeans located in said first path between said optical recording mediumand said beam collimation means for converging said collimated emittedlaser beam on said recording track; a first beam guide means locatedbetween said beam collimation means and said first converging means forguiding a portion of reflected laser beams from said optical recordingmedium through said first converging means in a second direction toproduce a first guided laser beam having a second path; a secondconverging means having a focal point located in said second path forconverging said first guided laser beam on said focal point; a secondbeam guide means located in said second path between said first guidemeans and said second converging means for guiding a portion of saidfirst guided laser beams in a third direction to produce a second guidedlaser beam having a third path; a first photodetection means located insaid third path for receiving said second guided laser beam to producetracking signals and information signals based on said second guidedlaser beam received thereon, said first photodetection means formed bydividing a photoelectric element into four portions vertically and intotwo portions horizontally such that eight segments of said dividedphotoelectric element are arranged in a matrix, said matrix formed in arectangular shape, each of said eight segments producing an electricsignal based on said second guided laser beam received thereon; a secondphotodetection means located in said second path between said secondconverging means and said focal point, said second photodetection meansreceiving said first guide laser beam to produce a first focus signal; abeam dividing means located in said second path between said secondconverging means and said second photodetection means for guiding aportion of said first guided laser beam in a fourth direction to producea third guided laser beam having a fourth path; a third photodetectionmeans located in said fourth path far away from said focal point, saidthird photodetection means receiving said third guided laser beam toproduce a second focus signal; and a signal reproduction means connectedwith said first photodetection means for reproducing said record signal,said signal reproduction means comprising:(a) a first addition means foradding signals produced by four of said eight segments arranged inopposite vertical side end portions of said matrix to produce a firstadded signal; (b) a second addition means for adding signals produced byfour of said eight segments arranged in center portions of said matrixto produce a second added signal; (c) a multiplier means for multiplyingsaid second added signal by a predetermined value to produce a firstmultiplied signal; and (d) a third addition means for adding said firstadded signal and said first multiplied signal to produce said recordsignal.
 2. An optical disk apparatus as claimed in claim 1 furthercomprising a tracking signal processing means connected with said firstphotodetection means for producing said tracking error signal, saidtracking signal processing means comprising:a fourth addition means foradding signals produced by four of said segments arranged on a left halfportion of said matrix to produce a first tracking signal; a fifthaddition means for adding signals produced by four of said segmentsarranged on a right half portion of said matrix to produce a secondtracking signal; and a subtraction means for obtaining a differencebetween said first and second tracking signals to produce said trackingerror signal.
 3. An optical disk apparatus as claimed in claim 2,wherein said fourth addition means adds signals produced by two of saidsegments vertically aligned on one side thereof opposite to said righthalf portion to produce said first tracking signal, wherein said fifthaddition means adds signals produced by two of said segments verticallyaligned on one side thereof opposite to said left half portion toproduce said second tracking signal.
 4. An optical disk apparatus asclaimed in claim 2, wherein said fourth addition means adds signalsproduced by two segments vertically aligned on one side thereof awayfrom said right half portion to produce said first tracking signal,wherein said fifth addition means adds signals produced by two ofsegments vertically aligned on one side thereof away from said left halfportion to produce said second tracking signal.
 5. An optical diskapparatus for reproducing an information recorded on a recording trackof an optical recording medium comprising:a light source means foremitting a laser beam having a first path in a first direction; a beamcollimation means located in said first path for collimating saidemitted laser beam; a first converging means located in said first pathbetween said optical recording medium and said beam collimation meansfor converging said collimated emitted laser beam on said recordingtrack; a first beam guide means located between said beam collimationmeans and said first converging means for guiding a portion of reflectedlaser beams from said optical recording medium through said firstconverging means in a second direction to produce a first guided laserbeam having a second path; a second converging means having a focalpoint located in said second path for converging said first guided laserbeam on said focal point; a second beam guide means located in saidsecond path between said first guide means and said second convergingmeans for guiding a portion of said first guided laser beams in a thirddirection to produce a second guided laser beam having a third path; afirst photodetection means located in said third path for receiving saidsecond guided laser beam to produce tracking signals and informationsignals based on said second guided laser beam received thereon, saidfirst photodetection means formed by dividing a photoelectric elementinto four portions vertically and into two portions horizontally suchthat eight segments of said divided photoelectric element are arrangedin a matrix, said matrix formed in a rectangular shape, each of saideight segments producing an electric signal based on said second guidedlaser beam received thereon; a second photodetection means located insaid second path between said second converging means and said focalpoint, said second photodetection means receiving said first guide laserbeam to produce a first focus signal; a beam dividing means located insaid second path between said second converging means and said secondphotodetection means for guiding a portion of said first guided laserbeam in a fourth direction to produce a third guided laser beam having afourth path; a third photodetection means located in said fourth pathfar away from said focal point, said third photodetection meansreceiving said third guided laser beam to produce a second focus signal;and a signal reproduction means connected with said first photodetectionmeans for reproducing said record signal, said signal reproduction meanscomprising:(a) a first addition means for adding signals produced byfour of said eight segments arranged in opposite vertical side endportions of said matrix to produce a first added signal; (b) a secondaddition means for adding signals produced by four of said eightsegments arranged in center portions of said matrix to produce a secondadded signal; (c) a multiplier means for multiplying said first addedsignal by a predetermined value to produce a second multiplied signal;and (d) a third addition means for adding said second added signal andsaid second multiplied signal to produce said record signal.